There is no use case in which one would need to share libdill's epoll
file descriptor with a child process. Even if it was innocuous, it makes
automated checking difficult as one has to consider this special case.

Since libdill does not provide access to the internal epoll fd, the only
way to fix this is to change the source code itself.

This is necessary to avoid leaking file descriptors to child processes.

The implementation tries to use accept4 when available, since that sets the
flag atomically with the creation of the connected socket, and falls back
to accept()+fcntl if not.

The IPC and TPC subsystems call dill_fd_own which does not preserve the
file descriptor flags. This means that even if the user creates the sockets
with the proper flags, when they use "fromfd" the file descriptors will still be
leaked to child processes.

The implementation tries to use F_DUPFD_CLOEXEC if available.